1. Field of the Invention
The present invention relates to chemical mechanical polishing of substrates, and more particularly to wear resistant retainers that restrict lateral movement of substrates during polishing without damaging the substrate and provide improved edge exclusion.
2. Background of the Art and Summary of Invention
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semi-conductive or insulative layers. After each layer is deposited, the layer is typically etched to create circuitry features. As a series of layers are sequentially deposited and etched, the uppermost layer on the substrate, i.e., the exposed surface of the substrate, may become nonplanar across its surface and require planarization. This occurs when the thickness of the layers formed on the substrate varies across the substrate surface as a result of the nonuniform geometry of the circuits formed thereon. In applications having multiple patterned underlying layers, the height difference between the peaks and valleys becomes even more severe, and can approach several microns.
Chemical mechanical polishing (CMP) is one accepted method of planarization. In a typical CMP system as shown in FIG. 1, a substrate 12 is placed face down on a polishing pad 14 located on a large rotatable platen 16. A carrier 18 holds the substrate and applies pressure to the back of the substrate to hold the substrate against the polishing pad during polishing. A retaining ring 20 is typically disposed around the outer perimeter of the substrate to prevent the substrate from slipping laterally during polishing. During polishing, a slurry comprising a reactive agent and abrasive particles is delivered to the polishing pad to chemically passivate or oxidize the film being polished and abrasively remove or polish off the surface of the film. A reactive agent in the slurry reacts with the film on the surface of the substrate to facilitate polishing. The interaction of the polishing pad, the abrasive particles, and the reactive agent with the surface of the substrate results in controlled polishing of the desired film.
The carrier uses a retaining ring in order to restrict lateral movement of the substrate as it is pressed against the polishing pad. The rotation of the polishing pad produces a substantial frictional force on the substrate surface that urges the substrate in the direction of the rotation. Therefore, it is inherent in the design of conventional CMP devices that the substrate will come into intimate contact with the retaining ring. Contact between the substrate and retaining ring can occur under significant force and can also involve some friction or impact. Under these conditions, the substrate edge can become cracked, chipped or otherwise damaged.
While the inner surface of the retaining ring contacts the substrate, the lower face of the retaining ring is in intimate contact with the polishing pad and the slurry. Contact with the polishing pad results in erosion of the retainer""s lower surface, thereby requiring frequent replacement. As an example, one material presently used in the construction of retainers is DELRIN(copyright), a registered trade name of E.I. DuPont Co. Retainers made of DELRIN(copyright) typically wear out after processing between about 300 and about 500 substrates, thereby requiring frequent replacement due to their high wear rate. In addition to high wear rates, another concern regarding retainers is the generation of potentially harmful particles in the system which may damage or scratch the substrate and compromise the integrity of devices formed on the substrate.
Therefore, there exists a need to characterize and provide a retaining ring and a material for its composition that is resistant to both the chemical and mechanical attack of the CMP environment and that does not damage the polishing pad. It would be desirable if the material was resistant to the mechanical forces of polishing, but would not adversely affect the edge of the substrate or compromise the integrity of the polishing pad. Additionally, the material also should be chemically compatible with the slurry composition so that the material will not break down in the polishing environment.
One such material for retainers is a ceramic, with a particularly preferred material being alumina. The inventors have found that alumina is wear resistant, has low particle generation and preserves the integrity of the polishing pad. However, alumina is an extremely hard material, i.e., does not exhibit any significant resilience, and can easily cause chipping or scratching of the substrate edge or surface. Furthermore, even though the alumina wear rate is low, the alumina itself is brittle and can chip or break upon impact or friction with the substrate.
Therefore, there exists a need for a wear resistant retaining ring that prevents damage to the substrate, is resistant to both the chemical and mechanical attack of the CMP environment, does not damage the polishing pad, and does not chip or otherwise damage the substrate on contact. It would be desirable if the retaining ring were made of materials that were resistant to the chemical and mechanical attack of the chemical mechanical polishing environment, and would protect the edge of the substrate and maintain the integrity of the polishing pad.
The present invention generally provides a retainer, e.g. a retaining ring for use in chemical mechanical polishing. The retaining ring includes a liner or insert formed or located on the inner diameter of the retaining ring to prevent chipping or scratching of the substrate edge. Preferably, the ring is made of a ceramic, such as alumina, and the liner or insert is made of a polymer that is resistant to the CMP environment. The preferred polymer liner or coating is comprised of an epoxy, polyimide, polyamideimide, polyetherketone or mixtures thereof, with the most preferred liner comprising a coating of epoxy.